Receivers for pulsed signals

ABSTRACT

In receivers of pulsed signals of a predetermined duration Tau , the useful signal is elaborated as a trigonometric function of the phase angle phi between the sum signal Sigma and difference signal of two samples, A, and B, of a received pulse at respective instants differing by t, t being smaller than Tau .

United States Patent Poinsard et al.

RECEIVERS FOR PULSED SIGNALS Henri Poinsard; Marie-Jacques Jullien, bothof Paris, France Thomson-CSF Apr. 22, 1970 Inventors:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data 7 Apr. 28, 1969 France 69/13415 U.S.Cl ..325/476, 325/323, 325/324 Int. Cl ..H03lr 9/04, H0413 l/ 10 Fieldof Search ..325/323, 324, 476

[451 Mar. 21, 1972 L .l i V.

UNITED sures PATENTS I 3,423,682 1/1969 Cauchois ..32s/324 3,177,4894/19'65 Saltzberg ..325/476 FOREIGN PATENTS OR APPLICATIONS 1,389,0681/19es France ..;.....32s 6s Primary Examiner-Howard W. BrittonAttorney-Edwin E. Greigg 57 ABSTRACT In receivers of pulsed signals of apredetermined duration 1, the useful signal is elaborated asatrigonometric function of the phase angle (/1 between the sum signal 2and difference signal of two samples, A, and B, of a received pulse atrespective instants differing by t, t being smaller than 1'.

9 Claims, 8Drawing Figures RECEIVERS FOR PULSED SIGNALS The presentinvention relates to receivers designed for the detection of pulse-typerecurrent radio signals, in particular radar receivers, in which theeffective signals are as picked up mixed with noise components and otherparasitic signals.

In known art, after amplification, possible frequency conversion,filtering and detection, the signals are generally integrated that is tosay that the signals, supplied by the video circuits at intervals oftime spaced by T, T being the periodicity of recurrence of the effectivepulse signals, are added up, this with a greater or lesser degree ofaccuracy.

This integrating operation has the effect of reducing the level of themajority of the parasitic signals in relation to that of the effectivesignals, the former generally not being recurrent or, if they are,generally not having the same recurrence frequency as the effectivesignals.

Among the signals obtained or integrated signals," there are, however,parasitic signals of substantial amplitude still left, in particularnoise signals.

These parasitic signals constitute false pieces of information, the meannumber of which per unit time must of course be limited if practicaloperation is to be preserved. This limitation is obtained by bottomclipping the signals produced at the output of the integrator.

All things being equal, the lower the clipping threshold, the higher theprobability of a false alarm; however, the threshold should be regulatedto the minimum value permissible in order to avoid excessive losses interms of weak effective signals.

The mean amplitude of these false elements of information being anessentially random factor, it is necessary to regulate the thresholdconstantly or to provide automatic gain control.

In either case, the rapidity of the variations in this amplitude do notalways make it possible to keep the probability of false alarm constant.

The invention relates to a radio receiver in which the false alarmincidence is statistically constant whatever the level of the signalspicked up.

According to the invention, there is provided a radio receiver for thedetection of recurrent pulse signals of predetermined duration 1-,comprising in series receiving means for receiving the recurrentsignals, integrating means for integrating the received recurrentsignals and clipping means for clipping the integrated signals, saidreceiver comprising further means inserted in series between saidreceiving means and said integrating means, for supplying a signal Uwhich is a finite trigonometric function of the phase apglg 45 betweentwcL vectors A and B defined as the resultants A =2 j Kand 2 j Kwhere Eis a vector representing the sum of the signals S (t) and S (t dt), S(t)and S (t+dt) representing the signals S received respectively at theinstants t and t+dt, and d! being of the same order of magnitude as 1',where Kis a vector representing the difference between the sigrials S(t)and S(t+dt) and wherej A is a vector deduced from A by rotation through11/2 in the trigometric sense.

In practice, simple functions will generally be adopted for U, eitherthe cosine or, preferably, the sine function.

The signal U being independent of the absolute value of the amplitudesof the signals S(t) and S(t+dt), the bottom clipping threshold can befixed.

The integrated signals corresponding to the random parasitic signalswill be weaker in relation to the effective signals, the larger thenumber of periods of integration.

In practice, it is sufficient to effect integration over some few tensofperiods.

In receivers of the kind in which the signals S(t) and S(t+dt) arepresented at a sufficiently low intermediate frequency F the algebraicalsums S(t) S(t-l-dt) and S(t) S(t+dt) can be effected at intermediatefrequencies, either directly if f, is a multiple of l/dt, or else afterphase shift of one of the signals; the signal U can then be obtainedquite simply by applying to the two inputs of an amplitude phasedetector, respectively the two sum signals after limitation thereof ifthe signal U cos is desired, one of the sum signals being in additionphaseshifted by 1r/2 if the signal U sin 45 is desired.

If the signals 8(2) and S(t+d!) are on the other hand available at videofrequency, then it will be possible to respectively remodulate twoauxiliary carrier waves of the same frequency and phase, either directlyby the signals or by 8+8 and SS'. The signal U can then be obtained inthe manner described hereinbefore, using an amplitude phase detector.

The signal U sin cancels out for S(!) S(t+d!). However, if the signalS(t) has, as a function of T, a symmetrical envelope with a maximum onthe axis of symmetry, for example a substantially triangular envelope,the curve illustrating U as a function of t is an odd function of (tt,)cancelling out for t t,,, where t is the instantaneous value of t whichdefines the axis of symmetry. This curve, in the neighborhood of zero,has a shape similar to the error curve of a range tracking system theconstant alarm probability receiver in accordance with the inventionthus exhibits the interesting additional feature that it is utilizable,without any supplementary arrangements, for the range tracking oftargets to this and the output signal from the integrator can be used.

The invention and its various applications will be better understoodfrom a consideration of the ensuing description and by reference to thefigures in which:

FIG. 1 illustrates an example of the shape of the signal detected by areceiver in accordance with the invention.

FIG. 2 is an explanatory vector diagram.

FIGS. 3 and 4 illustrate examples of signals produced in the receiver inaccordance with the invention.

FIG. 5 is the general basic diagram of a receiver in accordance with theinvention.

FIG. 6 is a diagram showing details of FIG. 5, in the context of oneapplication of the invention.

FIG. 7 is a diagram showing details of FIG. 5, in the context of anotherapplication of the invention.

FIG. 8 is an example of application of the invention in anelectromagnetic detection receiver.

By way of example, the invention will be described in the context of areceiver for an electromagnetic detection system which transmitssubstantially rectangular waveform pulses of width 1' at a constantrecurrence frequency F,. The receiver must therefore detect from amongall the signals received, those which are in fact echoes of thetransmitted pulses. The targets returning these echoes are illuminatedby the transmitter beam for a predetermined time T, which is a functionof the beamwidth and also of the target width if the latter is notnegligible. The N T F, echos from a target are processed in a filteringdevice, the internal characteristic of which more or less approachesthat of the optimum filter. In a conventional receiver, integrationduring the time T produces a signal whose power is compared for eachrange quantum, with an adjustable threshold. An echo is defined ashaving been received, when this threshold is exceeded.

However, the mean background noise level varies during the course ofoperation and accidental noise effects of external origin are frequentlysuperimposed upon it so that the probability of false alarms will varysince the threshold is fixed at least during the time of the recurrence.

In the receiver in accordance with the invention, the output signal fromthe optimum filter" is not directly integrated. An auxiliary signal U isproduced which is a function of the relative values of the detectedsignals at instants differing by a predetermined time interval, but isindependent of their absolute values. This is the signal which isintegrated over N recurrences. If it stems from random parasiticsources, it is statistically zero (or at least very small): bottomclipping to the level of a fixed threshold is now justified, the meannoise being brought at any instant to a statistically constant level.

FIG. 1 illustrates as a function of the time interval t, the signal S(t)at the output of the optimum filter, this in the case where echoes ofrectangular wave form pulses, of duration 1', are involved.

Depending upon whether the filter produces a video signal or anintermediate frequency signal, the signal corresponds to one of theenvelopes ABC or ABC of width 2 'r or to the modulated carrier ofenvelope ABCB'.

Self-evidently, this representation is a theoretical one. In fact, theenvelope is not strictly V-shaped but has a more rounded form, theessential as far as the present dissertation is concerned, being that itshould be symmetrical, with zeros at the extremities and a maximum onthe axis of symmetry.

The shape of this signal remains substantially the same for each of theN echos from one and the same target. Its amplitude is a function ofthat of the input signal to the filter.

A noise signal or more generally a parasitic signal, can randomly giverise during a recurrence, at the output of the filter, to a similarsignal but it will not generally be reproduced during anotherrecurrence.

First of all, the invention will be explained assuming that theprocessed signal is at video frequency. It will be shown hereinafter howthe invention may equally be applied if the signal is an i.f. signal.

Let S S(t) be the value of the signal at the time t and S S (1+dt) itsvalue at the time 1+ dt, dt being of the same order ofmagnitude as 1-.

If we call 2 the sum S+S, A the difference S-S', then the auxiliarysignal U produced in accordance with the invention is a function oftrigonometric f un c tions 9f the an le (1: between two vectorsA=2jAandB=2-jA ,where 2'andjAare two orthogonal (at right angles) vectors ofamplitude Z and A respectively, as FIG. 2 shows.

The functions chosen by preference, are the functions sin (1) and cos4:, these being obtained in a very simple manner as we will be seenhereinafter, and in particular the function sin d: which has specialadvantages.

FIGS. 3 and 4 represent cos 5 and sin (b as a function oftime in thecase where dt -r, the signals S(t) having the form shown in FIG. 1. Thesignal cos is null for r/2.

For orders sake, should be pointed out that the curves representing sindz and cos d) as a function of time, have been plotted taking, as originof the abscissae, the instant at which the two values S and S are equal,that is to say the samples have been taken symmetrically in relation tothe axis of symmetry of the signal.

In the case under consideration, sin 4) and cos d) are respectivelyexpressed by and at a later point in this discussion the generalexpressions for sin 45 and cos 11: will be given.

Even from these simplified expressions, it can already be seen that:

the signal sin 4) is an algebraic signal in all cases, the signal cosonly exhibiting two distinct polarities ifS and S do, in other words iftwo-polarity video working is used;

over several recurrences, the mean value of the signal sin 5 is zero forthe parasitic signals, whatever they may be in other words, S, and S, ofS and S for parasitic signal only are by definition identical so thattheir difference is zero;

over several recurrences, if the video signal is a two-polarity one, themean value of cos 4: is zero for the thermal noise, this noise beingen..irely decorrelated at the end of the time 1- however, it may not bezero for certain other parasitic signals.

The indicated approach, therefore, is to process the signal sin (b.

The effective signal will thus be obtained by the integration of the Nsignals corresponding to N successive recurrences for the echos the samesignal will appear N times at the integrator input,-and the signaltwo-noise ratio is thus improved overall in the ratio V N.

In all cases, to the extent that the spectrum width of the noise is atleast equal to the passband of the receiver, the integrated signal willin relation to the parasitic signals have a statistically constant meanvalue which makes it possible to fix the threshold once and for all.

lf,,on the other hand, the spectrum width of the parasitic signals issmaller than the passband of the receiver, N, being 5 the number ofnon-independent samples of parasitic signals (where N, is less than N),the ratio in effective signal to parasitic 7 signal after integrationwill be in the order of N N oth at the false alarm factor, for the samethreshold, will be increased this can be remedied either by increasingthe 10 time of illumination of the target in the ratio N/N or by using atransmitter-receiver system of rapidly varying frequency which meansthat the parasitic signals can be decorrelated from one recurrence tothe next.

FIG. 5 illustrates the fundamental diagram of a receiver in accordancewith the invention.

The signals picked up by the antenna A and possibly processed in themanner indicated in an input unit 51 or receiver proper, are applied toa device 52 which at the instant t dt produces at its output 521 thesignals received at the instant t, and at its output 522 the signalsreceived at the instant t dt thus, at the outputs 521 and 522 therespective signals S and S are obtained. These signals are combined inthe operator 53 which produces at two outputs 531 and 532 The phasebetween these latter signals is measured in a phase detector 54 whichproduces the signal U as a function of this phase.

'At 55, the signals U are integrated, these appearing at the recurrenceperiodicity of the transmitter of the electromagnetic detection systemin question the integrated signal is bottom clipped at 56 and at 57, theeffective signal, of statistically constant false alarm factor, isobtained.

The above described diagram is a highly general one.

Depending upon the form in which the signals are supplied at the output51, and thisessentially depends upon the nature of said device whichdoes not form part of the invention, the following devices will bedesigned in different ways.

The receiver unit 51 may produce a video signal in which case thedetailed diagram is that of FIG. 6 the unit 52 has two sampling devices61, 62 which are set in operation a time interval dt one after the otherby a clock 63, at the recurrence frequency of the system, and a delay64, whose characteristic delay is dt, or r in the example described ismounted, in series with one of the sampling devices.

The unit 53 has two modulators 65 and 66 coupled to one and the sameoscillator 67, the latter operating for, example, at 2 mc./sec., themodulators being followed by a conventional operator 68 comprising acombination of resistors and capaci- 5 tors and forming the sums S (l+j) S (l-j) and S (1j) S In the preferred case, where U sin (I: the unit54 has two identical limiting amplifiers 609 and 610, one of themfollowed or preceded by a 1r/2 phase shifter 611, and an amplitude-phasedetector 612 whose output supplies the integrator which latter canadvantageously be designed in known fashion as a loop containing atwo-input adder 613, one input of which is coupled to the output of thedetector 612 whilst the output is coupled to one of the ends of thedelay line 614 of characteristic delay T= l/Fr, the other end of thedelay line being coupled both to the bottom clipper 56 and to the inputof an amplifier 615 whose gain is close to but less than unity and whoseoutput is coupled to the second input of the adder.

Alternatively, the receiver unit may produce an intermediate frequencysignal in which case the device 52 may simply comprise, as FIG. 7 shows,a delay device 71 for bringing into time coincidence the signalsreceived at the instants t and t dz, and, if need be, a device 72 forphase control, possibly with a feedback arrangement, the carrierfrequency of the signals not generally being a'multiple of l/dt. In thiscase, the device 53 will simply comprise the operator 68, directlysupplied with the output signals from the device 52.

If the signals are not brought exactly into phase, the expressions forsin 1 and cos I become:

LII

the denominators only being identical to S S if S and S are in phasethese signals have the same advantages as the signals previouslymentioned as far as the elimination of parasitic components isconcerned.

The diagram of FIG. 7 is a very general diagram.

In FIG. 8, the receiver of a pulse-type electromagnetic detection systemwith fixed echo elimination and range channels, has been illustrated.This receiver is assumed to be of the coherent type, i.e., in which theechoes processed either stem from transmitted pulses, obtained by thechopping of one and the same carrier, or are detected using as areference signal of each of them, a signal which is in phase with thecarrier of the transmitted pulse. The receiver thus, in the conventionalway, comprises beyond the circuit 51, which in this case producesintermediate or video frequency signals, a number M of parallel rangechannels the input gates 81.l,81.2,81.M ofwhich are successively openedduring adjacent time intervals of duration 1, by opening pulsessynchronized by the general synchronizing device of the system, 200.

Each of these channels comprises a fixed-echo rejection filter 83.183.2.... 83M.

The output signals from two successive channels being, offset by 1 thechannels of successive outputs are grouped two by two and the outputsignal from the rejection filter for a channel thus constitutes both thesignal S of said channel and the signal S of the next channel (M-l)operators 68,, such as the operator 68 of FIG. 7, in other words 68.1,68.2.. 68 (Ml), have their inputs respectively coupled to the outputs ofthe filters 83.i and 83 (i+l respectively.

If the signals at the output of the device 51 are at intermediatefrequency, (M-l) devices 52.], 52.2, 52 (Ml) identical to the device 52of FIG. 7, are placed beyond each operator.

Each operator produces at two respectiveoutputs 8, and 9,, the signals Aand B hereinbefore defined.

The outputs 8, are sequentially coupled by the electronic switch 100,controlled by the device 200, to the limiting amplifier 69 similarly,the outputs 9, are coupled sequentially in synchronism with the outputs8,, by an electronic switch 101, to the limiting amplifier 620, which isidentical to the one already described. The two amplifiers are coupledas before to the amplitude-phase detector 612, one of them through a1r/2 phase shift element 611 if the function sin (1: has been chosen.

The output signal from the detector 612 is applied to the input of aswitch 102 with (M-l) outputs, synchronous with the preceding ones, sothat the integration ofthe signals U corresponding respectively todifferent range channels be effected correctly. To this end, each outputof the switch 102 is coupled to an integrating device 55, preceded by amemory device 12,.

In order to determine whether or not there is an effective echo present,the signal U of each channel will be compared in 56 with thepredetermined threshold, a further switch 103 being placed between 56and the outputs of the integrators 55,.

In the case where the signals at the outputs of the rejection filtersare detected, or correspond to Doppler frequencies which are very weakor differ two much from one another, for proper operation of thephase-detector circuit it will be necessary as in the case of FIG. 6, tosample the signals and to modulate an auxiliary carrier with thesesignals. Each range quantum will thus have to be sampled at timeintervals in the order of magnitude of the time of decorrelation of thethermal noise as appearing at the outputs of the rejection filters,i.e., time intervals of less than T= HP, in the normal case where thepassband of a rejection filter is small in relation to the recurrencefrequency F By way of example, for T= 200/usec. and M 20, themeasurement of each of the channels takes place during a time of lessthan lO/psec. and the frequency of the auxiliary wave can be made equalto 2 mc./sec.

The field of application of the invention is extremely wide since itcovers all the cases of reception of recurrent pulse signals which aremixed with parasitic noise components, that is to say in particular itapplies to numerous kinds of receivers used in aerial navigation and forspace surveillance purposes.

It should be pointed out too that it does not merely apply to constantrecurrent signal receivers in other words, the fundamental requirementis that the signals picked up from one and the same origin should bepresented in accordance with a known law of repetition.

This is in particular the case of the kind of signal known by the nameof bi-recurrent," currently utilized in electromagnetic detectionsystems in order to exclude blind zones."

Finally, it should be pointed out that in the case of a mobile rangegate system for target tracking application, the invention at the sametime makes it possible to effect acquisition of the target under goodconditions since the false alarm ratio is constant and therefore thepresence of echo" factor is optimized in order to effect tracking, it isthen sufficient, once the presence of an echo has been indicated by asignal at the terminal 57, to process the signal sin 45 picked up at theinput to the clipper 56, in the form of an error signal relating to thefeedback loop controlling the displacement of the window, the latterbeing correctly positioned when sin d) 0.

Of course, the invention is not limited to the embodiments described andillustrated here purely by way of example. In particular, the delay dtcould be other than 7.

What is claimed, is:

1. A radio receiver, for the detection of recurrent pulse signals S ofpredetermined duration 7, comprising in series receiving means forreceiving the recurrent signals, integrating means for integrating thereceived recurrent signals and clipping means for clipping theintegrated signals, said receiver comprising further means, inserted inseries between said receiving means and said integrating means, saidfurther means comprising a two input operator, means for applying tosaid two inputs the signals S S(t) and S' S(t dt), representing thesignals received respectively at the instants t and t dt, and a! beingof the same order of magnitude as r said operator having two outputsrespectively supplying the signals A=(S+S) +j(S-S') and B (S-i-S') j(SS') two limiting amplifiers having respective inputs coupledrespectively to the outputs of the operator, and an amplitude-phasedetector having two inputs coupled respectively to the outputs of saidamplifiers.

2. A receiver as claimed in claim 1 wherein, said receiver comprisesdetecting. means, said further means comprises two sampling devices forpicking pairs of samples of signal S the two samples of a pair beingtaken at instants which are spaced by said time dt, said samplingdevices having respective outputs, means for bringing into timecoincidence the two samples of each pair, an auxiliary oscillator,having an output, two modulators having respective oscillation inputscoupled to said oscillator outputs, respective modulation inputs coupledrespectively to said outputs of said sampling devices, and respectiveoutputs coupled to the inputs of said operator.

3. A receiver as claimed in claim 1 wherein said further means comprisesa direct channel and a delaying channel introducing a delay equal tosaid time dt said channels having respective inputs coupled to saidintegrating means and respective outputs respectively coupled to twoinputs of the operator.

4. A receiver as claimed in claim 3 wherein one of the said channelscomprises a phase-shifting device.

5. A receiver as claimed in claim 4 wherein a phase feedback devices iscoupled to the said channels for controlling said phase shift element.

6. A receiver as claimed in claim 6 for a pulse radar systemtransmitting recurrent pulses of duration 1' having a substantiallyrectangular waveform, said receiver comprising a filter device whichconverts the signals of width 1' into signals with a substantiallyVhshaped envelope of width 21-1- M (M being an integer greater than 1)range gate channels, numbered 1 to M successively opened during adjacenttime intervals equal to -r each channel including a fixed echo rejectionfilter having an output, said receiver further comprising (M-l)operators, numbered 1 to M-l an operator numbered i having two inputsrespectively coupled to the filter outputs of the channels numbered iand i l said operators respectively having first outputs supplying saidsignal A and second outputs supplying said signal B a first synchronizedswitch successively coupling said first outputs of said operators to oneof said limiting amplifiers, a second switch, operating in synchronismwith said first switch for successively coupling said second outputs ofsaid operators to the other limiting amplifier; integrator circuitsnumbered 1 to M-l the-integrator numbered 1' being associated with thechannels of order i and i+ l a third switch, synchronously operated withthe former two, coupling the output of said phase-detector successivelyto the (M-l) integrators; a fourth switch successively coupling theoutputs of the phase-detector successively to the (M-l integrators; anda fifth switch successively coupling the outputs of the integrators tothe clipping means.

7. A receiver as claimed in claim 6, designed to cooperate with apulse-type electromagnetic detection system comprising a sliding rangegate having a control input; said receiver comprising means forelaborating signal U sin #1 and for feeding said signal U to saidcontrol input.

8. A receiver as claimed in claim 6, in which the output signals of therejection filters are if. signals, further comprising, between theoutputs of two successive rejection filters of order i and i l and theinputs of the operator of order i respective (M-l delaying phaseshifting devices.

9. A receiver as claimed in claim 8, in which the signals at the outputof the rejection filters are video frequency signals, further comprisingsampling devices respectively coupled to the output of said rejectionfilter.

k II

1. A radio receiver, for the detection of recurrent pulse signals S ofpredetermined duration Tau , comprising in series receiving means forreceiving the recurrent signals, integrating means for integrating thereceived recurrent signals and clipping means for clipping theintegrated signals, said receiver comprising further means, inserted inseries between said receiving means and said integrating means, saidfurther means comprising a two input operator, means for applying tosaid two inputs the signals S S(t) and S'' S(t + dt), representing thesignals received respectively at the instants t and t + dt, and dt beingof the same order of magnitude as Tau , said operator having two outputsrespectively supplying the signals A (S+S'') + j(S-S'') and B (S+S'') -j (S-S'') , two limiting amplifiers having respective inputs coupledrespectively to the outputs of the operator, and an amplitude-phasedetector having two inputs coupled respectively to the outputs of saidamplifiers.
 2. A receiver as claimed in claim 1 , wherein, said receivercomprises detecting means, said further means comprises two samplingdevices for picking pairs of samples of signal S , the two samples of apair being taken at instants which are spaced by said time dt, saidsampling devices having respective outputs, means for bringing into timecoincidence the two samples of each pair, an auxiliary oscillator,having an output, two modulators having respective oscillation inputscoupled to said oscillator outputs, respective modulation inputs coupledrespectively to said outputs of said sampling devices, and respectiveoutputs coupled to the inputs of said operator.
 3. A receiver as claimedin claim 1 , wherein said further means comprises a direct channel and adelaying channel introducing a delay equal to said time dt , saidchannels having respective inputs coupled to said integrating means andrespective outputs respectively coupled to two inputs of the operator.4. A receiver as claimed in claim 3 , wherein one of the said channelscomprises a phase-shifting device.
 5. A receiver as claimed in claim 4 ,wherein a phase feedback devices is coupled to the said channels forcontrolling said phase shift element.
 6. A receiver as claimed in claim6 for a pulse radar system transmitting recurrent pulses of duration Tauhaving a substantially rectangular waveform, said receiver comprising afilter device which converts the signals of width Tau into signals witha substantially V-shaped envelope of width 2 Tau Tau , M (M being aninteger greater than 1) range gate channels, numbered 1 to M ,successively opened during adjacent time intervals equal to Tau , eachchannel including a fixed echo rejection filter having an output, saidreceiver further comprising (M-1) operators, numbered 1 to M-1 , anoperator numbered i havIng two inputs respectively coupled to the filteroutputs of the channels numbered i and i + 1 , said operatorsrespectively having first outputs supplying said signal A and secondoutputs supplying said signal B , a first synchronized switchsuccessively coupling said first outputs of said operators to one ofsaid limiting amplifiers, a second switch, operating in synchronism withsaid first switch for successively coupling said second outputs of saidoperators to the other limiting amplifier; integrator circuits numbered1 to M-1, the integrator numbered i being associated with the channelsof order i and i + 1 ; a third switch, synchronously operated with theformer two, coupling the output of said phase-detector successively tothe (M-1) integrators; a fourth switch successively coupling the outputsof the phase-detector successively to the (M-1) integrators; and a fifthswitch successively coupling the outputs of the integrators to theclipping means.
 7. A receiver as claimed in claim 6, designed tocooperate with a pulse-type electromagnetic detection system comprisinga sliding range gate having a control input; said receiver comprisingmeans for elaborating signal U sin phi and for feeding said signal U tosaid control input.
 8. A receiver as claimed in claim 6, in which theoutput signals of the rejection filters are i.f. signals, furthercomprising, between the outputs of two successive rejection filters oforder i and i + 1 , and the inputs of the operator of order i ,respective (M-1) delaying phase shifting devices.
 9. A receiver asclaimed in claim 8, in which the signals at the output of the rejectionfilters are video frequency signals, further comprising sampling devicesrespectively coupled to the output of said rejection filter.